I can’t reproduce this personally, but I’m on macOS 11.5.2 so there is a chance that is the variable.
I wonder, can you think of any keyboard/snippet/hotkey type apps you have installed? Anything that might commandeer your keyboard in any way?
Also, what installer version of Obsidian are you on? If you’re still on an old installer, it might be worth installing fresh from the website.
Edit: I was able to reproduce this now… I’m not sure why. I tried typing what you typed, and adding <enter> which is an opening tag, made some weird side effects, like the white space in front of each line, like in your video. And now my links aren’t auto-completing either sometimes, but when it did, it jumped to the other viewport.